This has seemingly become a bit of a common problem, apparently. I honestly do not know the cause of it. There is honestly nothing special about running the plugin, and if you don't have problems with the verification on any other functions, you shouldn't have a problem there.

Are you running it in subscriber.cgi or admin.cgi? You should be running it in subscriber.cgi, but I don't think that that would cause this problem.

You can fix this by simply changing subscriber.cgi line 294 as follows:

OLD: else {&ip_wrong; exit;}
NEW: else {print "";}

This will remove one level of security (IP Check), however the admin area is password protected. Many people work from home or have service providers that have dynamic IP addresses so this can be annoying.